Various flat-panel displays are being developed to overcome defects of a Cathode Ray Tube (CRT) display, such as high weight and volume. Flat-panel displays include a Liquid Crystal Display (LCD), a Field Emission Display, a Plasma Display Panel (PDP) and a Light Emitting Display (LED).
Among the flat-panel displays, the LCD includes a TFT (Thin Film Transistor) substrate, color filter substrate and a liquid crystal layer. A plurality of liquid crystal cells which are arranged in an area defined by a plurality of data lines and a plurality of gate lines are formed on the TFT substrate. In addition, a TFT as a switch device is formed in each liquid crystal cell, and color filters are formed on the color filter substrates. The liquid crystal layer is formed between the TFT substrate and the color filter substrate.
The liquid crystal display device generates an electric field corresponding to data signals, and reproduces images by controlling the transmissivity of the liquid crystal layer. Herein, polarity of a data signal is reversed for each frame, for each column or for each dot in order to prevent a degradation which occurs by applying an electric field to the liquid crystal in a direction for a long time.
The liquid crystal display device forms a color image by mixing red light, green light and blue light provided from 3-color dots of red (R), green (G) and blue (B). However, in a general liquid crystal display device for displaying one sub-pixel using 3-color dots of red (R), green (G) and blue (B), light efficiency is deteriorated. Since color filter arranged in each sub-pixel of red (R), green (G) and blue (B) is penetrated about ⅓ of incident light, light efficiency is reduced.
In order to maintain the color realization ratio and to improve the light efficiency in the LCD device, Korean patent publication No. P2002-13830 (LCD device) and Korean patent publication No. P2004-83786 (Apparatus for driving of display device and method for driving thereof) discloses an RGBW type LCD device which includes a white color filter W as well as red, green and blue color filters.
FIG. 1 is a graph showing white brightness of each display device according to an average picture level displayed.
In a case of a small size cathode ray tube or a plasma display panel as a self-emitting type display device, when an average picture level is high as line ‘A’, the white brightness is low, whereas when the average picture level is low, the white brightness is high. On the other hand, in a case of a cathode ray tube above 30-inch diagonal size, when the average picture level is high as line ‘B’, the white brightness is low, whereas when the average picture level is low, the white brightness is high.
In addition, in a case of the liquid crystal display as a non-emissive display, since brightness is determined according to brightness of a backlight unit and transmissivity of liquid crystal, variation of brightness according to the average picture level does not occur. A RGBW type liquid crystal display for a television has high brightness as a line ‘C’ due to a white pixel. On the contrary, a RGB type liquid crystal display for a television has low brightness as a line ‘D’ in comparison to the RGBW type liquid crystal display. In addition, in a case of a RGB type liquid crystal display for a computer monitor, since it is a small size, the white brightness is as a line ‘E’.
Since the RGBW type liquid crystal display device can acquire high white brightness using the same backlight, in comparison to a general RGB type liquid crystal display device, it is capable of acquiring white brightness as high as that of a cathode ray tube by enlarging a dynamic range with respect to the white brightness.
However, when the width ratio of white pixels is high, the conventional RGBW type liquid crystal display device causes eye strain of a user due to excessively high white brightness. Particularly, when the RGBW type liquid crystal display device is used as a multi-function monitor serving as both a television and a monitor, since most of word-processing or internet-access environment adopts a white image as a background, it causes user eye strain.